Infection of cells with poliovirus results in a marked and rapid decrease in host cell protein synthesis, followed by translation of viral messenger RNA to yield only virus-specified polypeptides. The inhibition of cellular protein synthesis has been shown to occur at the initiation step of protein synthesis. Translational initiation factors prepared from infected cells are inactive in stimulating initiation of synthesis of cellular proteins. The defect has been localized to a partially purified initiation factor which contains the cap binding protein (CPB), identified by its ability to bind and be cross-linked to the cap group of cellular mRNAs. We plan to analyze the CBP from infected cells and to compare its biochemical and functional properties with that isolated from uninfected cells. We will determine whether the cap group is the structural feature of mRNA which is discriminated against by infected cell initiation factors. In addition, we are analyzing the mechanism of initiation of viral protein synthesis, since it occurs under conditions where host cell translation is restricted. Lastly, studies have been undertaken to analyze the synthesis of viral RNA, both in vitro and in vivo.